Make weaveDNS use the docker bridge IP instead of its own

This means the `--dns` setting for containers will work even if the
weavedns container is restarted (and assigned a new docker network
IP).

weave

@@ -46,6 +46,7 @@ MTU=65535
 PORT=6783
 HTTP_PORT=6784
 DNS_HTTP_PORT=6785
+DOCKER_BRIDGE=${DOCKER_BRIDGE:-docker0}
 
 COMMAND=$1
 
@@ -139,6 +140,11 @@ destroy_bridge() {
     run_iptables -t nat -X WEAVE >/dev/null 2>&1 || true
 }
 
+docker_bridge_ip() {
+    DOCKER_BRIDGE_IP=$(ip -4 addr show $DOCKER_BRIDGE | grep -o 'inet [.0-9]*')
+    DOCKER_BRIDGE_IP=${DOCKER_BRIDGE_IP#inet }
+}
+
 # the following borrows from https://github.com/jpetazzo/pipework
 
 # run args $2 $3 ... as a command in the network namespace of container $1
@@ -369,7 +375,8 @@ case "$COMMAND" in
         shift 1
         check_not_running $DNS_CONTAINER_NAME $DNS_IMAGE
         create_bridge
-        CONTAINER=$(docker run --privileged -d --name=$DNS_CONTAINER_NAME -v /var/run/docker.sock:/var/run/docker.sock $DNS_IMAGE "$@" | tail -n 1)
+        docker_bridge_ip
+        CONTAINER=$(docker run --privileged -d --name=$DNS_CONTAINER_NAME -p $DOCKER_BRIDGE_IP:53:53/udp -v /var/run/docker.sock:/var/run/docker.sock $DNS_IMAGE "$@" | tail -n 1)
         with_container_netns $CONTAINER attach $CIDR >/dev/null
         echo $CONTAINER
         ;;
@@ -407,7 +414,8 @@ case "$COMMAND" in
         CIDR=$1
         shift 1
         create_bridge
-        DNS_ARG=$(docker inspect --format '--dns {{ .NetworkSettings.IPAddress }}'" --link $DNS_CONTAINER_NAME:$DNS_CONTAINER_NAME" $DNS_CONTAINER_NAME 2>/dev/null) || true
+        docker_bridge_ip
+        DNS_ARG="--dns $DOCKER_BRIDGE_IP"
         DNS_SEARCH_ARG="--dns-search=."
         for arg in $@; do
            case $arg in